1. Field of the Invention
The present invention relates to an image pickup device and a camera module configured by using the image pickup device, and in particular, relates to signal processing for generating RGB signals based on an RGB color ratio correlation. For example, the image pickup device is utilized in a solid-state image pickup device such as a charge-coupled device (CCD) image sensor and a CMOS type image sensor, a cellular phone with an image sensor, a digital camera, a video camera and the like.
2. Description of the Related Art
Recently, pixels of an image sensor are expedited to be fine, and the pixel pitch of the 2 μm range is in practical use. Further, the pixel pitch of 1.75 μm and that of 1.4 μm are under development. With the fine pixel which pitch is equal to or less than 2 μm, S/N of image signals deteriorates because the amount of incident light at a receiving surface thereof is greatly reduced. Further, conventionally, a color camera has a problem that the image quality deteriorates due to a false color signal or a color noise which is caused by refractive index difference at the time of passing through a lens generated by light wavelength difference of RGB or the miniaturization of the pixel pitch. Regarding the suppression of the false color signal or the reduction of the color noise, a variety of measures have been proposed.
See, for example, Jpn. Pat. Appln. KOKAI Publication Nos. 4-235472, 2002-10108, 2005-303731, 2001-245307, and 5-168029.
However, a radical solution is not proposed in these measures.
Further, with a single plate color camera, for example, the image quality deteriorates due to the generation of an RGB color drift which is a false color at an edge portion of an object image or due to a dot-shaped color noise caused by a random noise of each RGB.
There are two generation factors of the false color signal. Regarding one factor, there is a generation of a beat at the vicinity of critical resolution of a sensor due to a mismatch between the pitch of the pixel and the pitch of the high resolution pattern of an object. Since the frequency of the beat is low, the false color signal is caused by mixing of a low frequency component into the frequency of the captured signal. Regarding the other factor, as mentioned above, the refractive indexes of RGB lights differ because of the wavelength difference of the RGB lights which are incident into an optical lens, and then, the false color signal is caused by chromatic aberration in magnification where the RGB images drift at the periphery area of the sensor.
As shown in FIG. 24, the chromatic aberration in magnification becomes large especially at the periphery area of the sensor. Further, as making the F value of the lens small in order to correspond to the fine pixel of the sensor and as the higher the spatial frequency corresponding to the pixel pitch becomes, the larger the chromatic aberration on the axis becomes even at the center of the lens due to the difference in decrease of modulation degree of a transfer function MTF (Modulated Transfer Function) of RGB lights. Therefore, with the fine pixel, the deterioration of the image quality is significant.
FIGS. 25A and 25B respectively show an obtained image when shooting red characters and the signal wave-shapes corresponding thereto. FIG. 25A shows a defocused image which is shot in the state that an objective lens is defocused and the signal wave-shape thereof. FIG. 25B shows an image with which the objective lens is focused and the signal wave-shape thereof. In the defocused image in FIG. 25A, the signal wave-shape corresponding to the edge portion of the displayed character gradually varies, and phases of a G signal and a B signal drift due to the chromatic aberration. Therefore, false color development occurs at the edge portion. This defocused image easily occurs especially with a fixed focus type objective lens.
FIG. 26 shows modulation degree characteristics of MTF in the condition that the focus position is adjusted at 1 m or longer with a fixed focus lens. As shown in FIG. 26, when the focus position is adjusted at a distance of 1 m or longer from the lens, the close object which is at the distance of less than 1 m cannot be focused. The shorter the distance becomes, the more the defocusing width increases. Then, signal defocusing occurs caused by the decrease of modulation degree and the chromatic aberration shown in FIG. 24. The defocusing width corresponds to the inclination amount of the signal wave-shape from the vertical position. On the contrary, when the focus is adjusted to a close object which positions, for example, at 30 cm from the lens, image defocusing occurs with the far object which positions at 1 m or farther from the lens. Further, even with the image where the object is focused, the chromatic aberration (the chromatic aberration in magnification) easily occurs at the periphery of the sensor, namely the periphery area of the lens, as shown in FIG. 24. Furthermore, as mentioned above, there is a case that false color development occurs due to the chromatic aberration on the axis even at the center of the lens.
In addition, in a prior art disclosed in WO2006095110, a solid-state image pickup device which deepens the depth of field is disclosed. However, there is a problem that a false color occurs to the image since no solution to the problems of the chromatic aberration of the lens is taken. Then, even if measures against the above-mentioned problem are taken, the device cannot be adapted to a variety of object conditions, and thus, efficient resolution and resolution feeling and a correct color replay image cannot be obtained.